How to keep a laptop running in a power cut

In a power cut, a laptop can run from four things besides its own battery: a USB-C power bank, a portable power station, the car, or a solar panel feeding one of the first two. Which of them works for you comes down to two numbers on the labels. Watts say how fast something can deliver power, and watt-hours (Wh) say how much it holds.

For scale, the battery in the M2 MacBook Air holds 52.6 Wh. A power bank sold as 25,000 mAh gives you about one more battery's worth, a small power station gives you around four, and a 100W solar panel adds a few hundred Wh on a decent day. The tables further down turn each of those into hours of use, with every step of the arithmetic shown, so you can swap in your own laptop and your own gear. The capacities and loss factors come from the makers' own pages (Anker and EcoFlow both sell this equipment) and we have measured none of it ourselves, so read the hours as planning figures.

Start with the battery the laptop already has

Apple lists a 52.6-watt-hour battery for the MacBook Air (M2, 2022) and rates it at up to 15 hours of wireless web use. That is a best case. Apple's footnote says the test browsed 25 popular websites with the display brightness set to 8 clicks from the bottom, and that battery life varies by use and configuration.

You can stretch what is there. Apple says Low Power Mode reduces energy use to increase battery life, and Microsoft describes the energy saver in current versions of Windows as a balance between performance and longer battery life. Microsoft's battery tips also include turning the display brightness down.

The FCC's emergency advice is to charge your devices when a storm is coming and to keep extra batteries, a solar charger or a car charger on hand. It also warns that charging a phone from your laptop over USB may drain the laptop's battery, so charge the phone from a power bank if you have one.

How many watts your laptop needs

The charger that came with the laptop has its output printed on it as volts and amps, and Anker's advice is to multiply the two to get watts. EcoFlow gives rough charger ranges by laptop type: 30 to 45W for lightweight laptops, 60 to 70W for productivity laptops and 90 to 100W for higher-performance ones. Apple recommends a 30W adapter for the 13-inch MacBook Air and 140W for the 16-inch MacBook Pro.

That figure is a ceiling. EcoFlow points out that charger wattage is the most the adapter can put out, not what the computer is pulling at any given moment, and the real draw changes by the minute.

A source with fewer watts than your charger is still useful. Apple says a USB-C Mac will charge from any USB-C power adapter or display, even one with higher or lower wattage than the recommended adapter, though the best experience comes from at least the recommended minimum. Anker describes a 65W laptop on a 30W car charger as one that may charge slowly, stop charging under load, or only hold its battery level. In a long outage, holding the level may be all you need.

At the top end, USB Power Delivery was limited to 100W for years, using 20V and cables rated at 5A. The 3.1 revision, announced in 2021, allows up to 240W. A power bank port rated for 100W can therefore feed anything in the 90 to 100W class, as long as the cable is rated for 5A.

Older laptops with a round barrel plug are a separate case. Anker says barrel-pin laptops may need a universal DC adapter with the correct tip, which is better sorted out before the lights go out.

USB-C power banks

Two things matter on a power bank's label: how many watts each USB-C port can deliver, and the capacity. Anker's 25,000 mAh laptop bank lists three USB-C ports of up to 100W each, so it clears any laptop up to the 100W class.

Capacity is usually printed in mAh, which cannot be turned into watt-hours without a voltage. Anker's conversion guide uses Wh = mAh × V ÷ 1,000, with 3.7 V for a power bank. For the 25,000 mAh bank that is 25,000 × 3.7 ÷ 1,000 = 92.5 Wh.

You will not get all of it. Anker's own FAQ for that bank says energy loss in the cells and circuitry cuts capacity by 30% to 45%, so a full 25,000mAh bank typically delivers about 13,750 to 17,500mAh. At 3.7 V the low end is 50.9 Wh, a shade under one MacBook Air battery. We use that low end in the tables below.

A power bank is a reserve. It cannot refill itself during a cut unless you also have a car or a solar panel to put energy back in. Ready.gov tells households to plan for batteries and other alternative power sources, with a portable charger or power bank as its example. Charge it now and top it up from time to time.

Portable power stations

A power station is a large battery with a wall socket and USB-C ports. EcoFlow's RIVER 2 holds 256Wh and its RIVER 2 Pro holds 768 Wh. They are the two examples in our laptop power station calculator, which also explains where the 0.85 below comes from. EcoFlow's planning rule is rated Wh × 0.85 ÷ the laptop's watts, because some energy is lost as heat on the way out.

EcoFlow adds that charging through a high-wattage USB-C port skips a conversion and can give slightly longer runtime, so the tables are a little cautious for a USB-C connection. It also notes that laptops pair well with power stations, since their needs sit well below those of heaters, kettles or cooking gear.

Of everything on this page, a larger power station is the only option that holds several days of light laptop use on its own.

Charging from the car

Most cars have a 12V accessory socket. Anker says its limit depends on the vehicle's fuse, with many sockets sitting around 120W to 180W, and tells you to check the manual before plugging in anything big. A basic 12V USB charger, the kind made for phones, is usually not powerful enough for a laptop and may charge very slowly or not at all. Anker's advice is a USB-C Power Delivery car charger at or above the laptop's wattage: a 65W laptop should ideally use a 65W or higher one. An inverter, which gives you a normal wall socket in the car, also works but adds conversion loss and can draw more from the car.

Engine on or off makes a difference. With the engine running, Anker says the alternator helps power both the car and the laptop. With the engine off, it advises against long sessions, because draining the battery deeply can leave the car unable to start.

Then there is the exhaust. The CDC says never to run a car or truck inside a garage attached to a house, even with the garage door open. The FCC gives the same warning for this exact use: if you are charging devices from your car without power at home, be careful and stay alert to carbon monoxide, especially in a closed space. Park outdoors.

Small solar panels

A panel does not usually plug into a laptop. EcoFlow says the energy must be converted and stored in a power station that supports solar charging, so in practice a panel tops up a station, and the station runs the laptop.

The wattage on the panel is a laboratory figure. EcoFlow's guide says to expect anywhere from 25W on a cloudy day to about 90W in bright summer conditions from a 100W panel. Over a whole day, what counts is peak sun hours. EcoFlow says most areas get 3 to 5 hours of peak sunlight daily. NREL puts the solar resource at 3.0 to 6.0 kWh per square metre per day over most of the continental US, and notes that one kWh/m2 is sometimes called a "sun-hour". It also gives losses between panel and usable power of about 15%, so we use 0.85 again.

For a 100W panel that is 100 × 3 × 0.85 = 255 Wh on a 3 sun-hour day and 100 × 5 × 0.85 = 425 Wh on a 5 sun-hour day. On a cloudy day, EcoFlow's 25W is less than a light laptop needs live, so the panel only trickles into the station's battery.

The arithmetic: watts, watt-hours and hours

Hours of use = usable watt-hours ÷ the laptop's watts. The three columns below use 30W (the bottom of EcoFlow's lightweight range), 65W (EcoFlow's own worked example) and 100W (the top of the higher-performance range). Most laptops sit well below their charger rating most of the time, so read the 65W and 100W columns as hard use.

First, how much each source holds once losses are taken off.

Then hours at each laptop load, rounded to one decimal.

The RIVER 2 row at 65W lands on 3.3 hours, the same answer EcoFlow gets in its own example. Last, what one 100W solar panel adds per day of sun.

Now a plan for an outage. Say you want the laptop for 4 hours a day over 3 days at 30W. That is 30 × 4 × 3 = 360 Wh. A RIVER 2 on its own falls short. A RIVER 2 Pro covers it with room to spare. A RIVER 2 plus one 100W panel works as long as you get a reasonable day of sun at some point, because the panel adds 255 Wh on a 3 sun-hour day. A power bank alone gives you about 1.7 hours at 30W per full charge.

When we searched for this on 25 October 2026, the first results came mostly from companies that sell power banks, power stations or solar panels. GoSun's guide (May 2024) covers the same routes and cautions against draining a car battery. Xtorm's (December 2024) says a large power bank can power a laptop. Neither says how long a laptop would run, so you cannot tell whether a given bank lasts an evening or a week. The advice is sound as far as it goes. The numbers are what we could not find, so we worked them out above.

What not to do

Generators are the main danger in a power cut. The CDC describes carbon monoxide as an odorless, colorless gas that kills without warning, and the CPSC says CO from a generator can kill you in minutes. The CDC says never to use a generator inside your home or garage, even with doors and windows open, and to keep it outside, more than 20 feet from any windows, doors and vents. Ready.gov gives the same distance and adds attached garages.

So do not run a generator in a garage, or a car in an attached garage, to charge a laptop. A power bank or a battery power station has no exhaust, which is why this page leans on them.

Before the next outage

1. Read the watts on your laptop's charger, or multiply its volts by amps.
2. Buy a power bank that ideally delivers at least that many watts from one USB-C port, with a cable rated for it.
3. Charge it, then charge the laptop from it for a few minutes while the laptop is unplugged from the wall, to see that the two work together.
4. Decide how many hours a day the laptop is for. Multiply by its watts and by the number of days, and compare the result with the Wh in the tables.
5. If the answer is more than a power bank holds, price a power station, and look at whether a panel fits your budget and your sky.

We're building Waystone, an offline preparedness assistant for laptops that is still in development and not for sale. It is designed to work without a connection, but it still needs a charged laptop, so the power question is ours too. Our guide to running AI offline on a Mac covers the software side, and the food timer covers what is in your fridge while you wait.